1. Field of the Invention
The present invention relates to a process for the preparation of certain phenylglyoxylic acid esters (some of which are known and others of which are new), which can be used as intermediates for the synthesis of herbicidally active compounds.
2. Discussion of the Prior Art
It has already been disclosed in Angewandte Chemie 68, 430 (1956) that phenylglyoxylic acid esters (I), for example phenylglyoxylic acid methyl ester or phenylglyoxylic acid ethyl ester, are obtained in accordance with equation 1 below when benzoyl cyanide (II) is hydrolyzed with a concentrated mineral acid such as, in particular, with concentrated hydrochloric acid. See also Berichte der deutsch chem. Gesellschaft 10, 429 and 844 (1877) and Organic Synthesis 24, 16 (1944). The phenylglyoxylic acid (III) thereby formed is isolated and esterified with an alcohol in the presence of a mineral acid, for example hydrochloric acid, sulphuric acid or p-toluenesulphonic acid. See Berichte der deutsch Chem. Gesellschaft 12, 629 (1879); J. Org. Chem. 29, 278 (1964); Chem. Abstracts 74, 99 587j (1971) and U.S. Pat. No. 3,754,006. ##STR3##
However, this process has a number of disadvantages. Thus, with respect to the benzoyl cyanide, it is a two-stage process. In the first stage, that is to say the hydrolysis of benzoyl cyanide, considerable amounts of benzoic acid are always formed, in particular when acids which are not sufficiently concentrated are used (see Angewandte Chemie 68, 430 (1956)). The total yields are only between 35 and 70%. The proportion of by-products, in particular benzoic acid esters, of up to 20% is accordingly very considerable. In addition, very long reaction times are required. When the process is applied on a laboratory scale the reaction times are between 12 and 20 hours.
Moreover, it is also known that carboxylic acid esters (IV) are obtained in accordance with equation 2 below from nitriles (V) in a "one-pot process" when the latter are converted, with alcoholic hydrochloric acid, into imide-ester hydrochlorides (VI), which are then easily hydrolysed by water into carboxylic acid esters and ammonium chloride (see Houben-Weyl, Methoden der organischen Chemie (Methods of Organic Chemistry), Georg Thieme Verlag Stuttgart 1952, volume VIII, page 536 et seq.). ##STR4##
However, this process, which is in itself very appropriate for the preparation of carboxylic acid esters, cannot be applied to the preparation of phenylglyoxylic acid esters, that is compounds of the formula (IV), in which R' is a benzoyl radical, or of the formula (VII) below. If benzoyl cyanides, that is compounds of the formula (V), in which R' is a benzoyl radical, or of the formula (VIII) below, are used as starting materials, the reaction does not take the desired course. Rather, benzoyl cyanides decompose with alcohols in an acid medium into benzoic acid esters and hydrocyanic acid (see Angew. Chemie 68, 430 (1956)).